The present invention relates to a core-shell type colorant-including fine particle and a polymer emulsion type water-based ink by use thereof having superior color reproduction quality and fastness.
In recent years, elimination of solvents or water-based constitution has been demanded with respect to recording materials and an inking material utilized for printers, printing machines, markers, writing tools, etc. Especially as water-based recording materials utilized for an ink-jet system, those comprising mainly an aqueous solution of a water-soluble dye and those comprising mainly a fine dispersion of a pigment having been commonly used.
 less than Recording Material Using Water-Soluble Dyes greater than 
In a recording material comprising water-soluble dyes, there is used an aqueous solution of water-soluble dyes which are categorized mainly into acid dyes, direct dyes and a part of food dyes, being added thereto glycols or alkanolamines as a wetting agent, surfactants or alcohols in order to adjust the surface tension, resins as a binder component and the like. The recording materials using water-soluble dyes are most commonly used because of the high reliability of anti-clogging at a pen top or in a recording system. The recording materials using water-soluble dyes are, however, easy to cause blotting on recording paper because they comprise an aqueous solution of dyes. Further, print quality is forced to deteriorate due to blotting of the ink which has been adjusted so as to quickly permeate into recording paper for accelerating the apparent drying speed. Water resistance is naturally poor due to the nature of water-soluble dyes. In addition, light fastness is very poor because water-soluble dyes only permeate into the recording material and they are dried to adhere in the recording paper. They are hardly said being dyed.
Heretofore, recent years, several techniques such as a technique disclosed in JP-A 2000-281947 (the term, JP-A refers to an unexamined and published Japanese Patent Application), in which dyeing ability is enhanced by utilizing an interaction between a dye and a polymer in a two-solution system with cationic polymers to prevent blotting and enhance fastness, have been tried, but it is hard to say that light fastness has been improved while keeping color reproduction quality of water-soluble dyes.
As a mean to overcome the aforementioned problems of recording materials using water-soluble dyes, addition of resin fine particles such as emulsions, latexes and the like has been studied for years. There proposed a recording material for ink-jet recording in JP-A 55-18418, in which a latex, xe2x80x9ca kind of colloidal solution comprising fine particles (having a particle diameter of approximately from 0.01 to a few xcexcm) of components such as rubbers and resins dispersed in water by use of emulsifiersxe2x80x9d is added. As latexes preferably used, listed are synthetic rubber latexes such as a styrene-butadiene copolymer latex, an acrylonitrile-butadiene copolymer latex, a polychloroprene latex, a vinylpyridine-stylene-butadiene latex, a butyl rubber latex, a polybutadiene latex, a polyisoprene latex and a polysulfide rubber latex; or synthetic resin type latexes such as an acryl ester type latex, a styrene-butadiene resin latex, a vinyl acetate type latex, a vinyl chloride type latex and a vinylidene chloride type latex.
Synthetic rubber type latexes listed in the above patent proposal, whose specific gravity is approximately within a range of from 0.9 to 1.0, satisfy such conditions to some extent, but there are problems in terms of light fastness and weather-proofing because most synthetic rubbers have unsaturated double bonds in the molecule. On the other hand, when the number of the unsaturated bonds is decreased by vulcanization, there occurs a problem of recording quality due to retardation of fixing of particles on recording paper. Further, excessive vulcanization causes a problem of precipitation because the specific gravity becomes not less than 1.1. In addition, the synthetic rubber type latexes have a low glass transition temperature to easily form film at room temperature, which causes ink-jet nozzles to be often clogged when the ink is dried at the top portion of the nozzles, and furthermore, the property of the dried substance being soft and slightly sticky making it very hard to be removed. The synthetic resin latexes listed in the patent all causes precipitation in a particle diameter range of exhibiting anti-blotting effect because the specific gravity reaches to over 1.1 and especially with the synthetic resin containing halogens it reaches to nearly 1.3 to 1.5.
Further, it can be said generally for these latexes that most of emulsifiers used for preparing the latexes tend to accelerate foaming of an ink and cause many problems due to unnecessary decrease of surface tension. In JP-A 54-146109 is proposed a recording material utilizing a water-soluble dye to which vinyl polymer fine particles having been swelled with solvents and dyed with oil dyes are added. There are listed mainly (metha)acrylic acid ester type copolymer fine particles as polymers suitably utilized and also there is described that a glass transition temperature of not higher than 30xc2x0 C. is a preferable condition. It is self-evident that fine particles having such a low glass transition temperature and having been swelled with solvents show film-forming property when they are dried at room temperature, and it can be easily analogized that utilizing such an ink will cause nozzle clogging frequently. There is a proposal of an image recording ink, in JP-A 3-56573, which contains dyes or pigments and 5 to 40 weight % of organic ultra-fine particles having an average particle diameter of not more than 0.5 xcexcm and being internally three-dimensionally cross linked. However, because such ultra-fine fine particles cannot be fixed on recording paper, it results in deteriorated durability of recorded images.
 less than Recording Material Using a Pigment Dispersion greater than 
In order to overcome the weak point of recording materials comprising water-soluble dyes, there is proposed a recording material comprising carbon black or an organic pigment. In such recording materials comprising a dispersion of pigments, water-resistance of an ink is markedly improved. The specific gravity of these pigments is, however, as high as 1.5 to 2.0, and caution is required in respect with precipitation of dispersed particles. To disperse pigments having such a high specific gravity stably, it is necessary to finely disperse so as to make an average particle diameter down to approximately 0.1 xcexcm, which makes the cost of dispersion very high resulting in to produce an expensive ink.
 less than Recording Material Using Colored Resin Particles greater than 
A proposal of dyeing water-dispersible resins by an oil-soluble (oleophilic or hydrophobic) dye is presented as an ink-jet ink. This is a proposal on xe2x80x9can ink using colored polymer fine particles as a recording materialxe2x80x9d. For example, in JP-A 54-58504, an ink in which a mixture of a hydrophobic dye solution and vinyl polymer fine particles is dispersed as an oil-in-water type is proposed. It is disclosed that vinyl polymer fine particles being mixed with hydrophobic dye solution are swelled by solvents of a dye solution and colored with a dye. It is also described that because of utilizing hydrophobic recording material, obtained images result in of water-resistant. In the proposal, by utilizing water as a continuous phase and colored vinyl polymer particles swelled with solvents as a dispersed phase, the control of ink viscosity is subjected to water allowing solvents having relatively a high viscosity (low volatile) to be used.
In JP-A 55-139471 and 3-250069, there is proposed an ink using emulsion polymerization or dispersion polymerization particles colored with dyes. The point of the proposal is to prevent blotting by use of colored particles as a dispersed phase and water as a dispersion medium, which is similar to that of JP-A 54-58504, but it is necessary to fix onto recording paper by film-formation of the particles because solvent is not included in the case of this proposal. Preferable particle diameter is suggested to be in a range of sub-micron, in terms of necessity of film-formation and secure of dispersion stability. In JP-A 54-58504, it is allowed to enhance dyeing ability by swelling of polymer fine particles with solvents, but, in this case, a problem of nozzle clogging occurs due to film formation by drying at the top portion of nozzles.
Further, in such recording materials comprising colored fine particles, the polymer composition has been mainly adjusted to enhance the stability thereof in an ink, but it has been very difficult for colored fine particles to achieve homogeneous incorporation of a colorant and stability at the same time.
As described above, a fine particle dispersion type recording material utilizing pigments, colored resins, conceal possibility of overcoming problems accompanied with a water-soluble dye type recording material and of achieving excellent recording quality, but still various kinds of problems are remained, and especially a conflicting problem, in which using dyes with superior color reproduction quality results in insufficient fastness while using pigments with excellent fastness results in poor color reproduction quality, has not been solved. Accordingly, the object of the present invention is to provide a water-based ink, which comprises core-shell type colorant including fine particles, having enhanced fastness and superior color reproduction quality.
The foregoing objects of the present invention can be accomplished by using a water-based ink, comprising a suspension of fine particles which comprise a polymer core containing a colorant and a polymer shell having a colorant content less than that of a core, and having a specific range of a particle diameter and a specific variation coefficient of a particle diameter, as described below.
(1) A polymer dispersion comprising water and polymer particles having a size of 5 to 500 nm in volume average diameter (or 50% volume particle diameter) and having a core-part and a shell-part in each of the polymer particles, wherein the core-part and the shell-part each comprise a colorant and a polymer, and the shell-part contains the colorant in an amount of not more than 80 weight % of the colorant in the core-part.
(2) The polymer particles of item (1), wherein the core-part and the shell-part each comprise a colorant and a polymer, and the shell-part contains the colorant in an amount of not more than 50 weight % of the colorant in the core-part.
(3) The polymer particles of item (1), wherein the variation coefficient of the volume diameter of the polymer particles is not more than 80%.
(4) The polymer particles of item (1), wherein the shell-part in each of the polymer particles contains the polymer in an amount of 5 to 95 weight % of the polymer in each of the polymer particles.
(5) The polymer particles of item (1), wherein each of the polymer particles contains the colorant in an amount of 20 to 1000 weight % of the polymer in each of the polymer particles.
(6) A water-base ink for ink-jet printing comprising the polymer dispersion of item (1).